1. Field of the Invention
The present invention relates to a circuit for detecting a trouble in electric lines connecting relatively movable circuits, and more particularly to a circuit for detecting insufficient electrical connection in a sliding contact mechanism connecting an external circuit disposed on a fixed construction to a circuit carried by a relatively movable body.
2. Description of the Prior Art
An electrical apparatus including a body movable relative to a fixed construction has a wide range of application. In such an apparatus, electrical connection or connections are made through sliding contact mechanisms, such as ring and brush mechanisms, for supplying electric power to an electric circuit provided on the movable body or for deriving signals therefrom to an external electric circuit disposed on the fixed construction.
Since such a sliding contact mechanism often causes such troubles as defective or insufficient contact or incapability of power supply which may result in a grave adverse effect on the electrical apparatus, it is required that the trouble be immediately detected and a suitable countermeasure taken.
As an example of the electrical apparatus, an electrophotographic copying apparatus is known in which electric power is supplied through a sliding contact mechanism to a heater element carried by a rotatable heating roller in the fixing section of the apparatus to heat up the roller and, at the same time, electric signals are also supplied from a thermosensitive element also carried by the roller through a further sliding contact mechanism to an external circuit for detecting the temperature condition of the roller. The present invention can be applied to such an electric control circuit having a sliding contact mechanism, and in particular to an improved temperature control circuit for preventing overheating of the heating roller of such a copying apparatus.
As is generally known, the electrophotographic copying apparatus has an electrostatic-latent-image-forming section, a developing section and a fixing section for heating and fixing a toner image formed on a sheet of paper to provide a desired picture. There are two types of heating devices, that is, an oven type device and a heating roller type device, for use in the fixing section of the copying apparatus. In the oven type device, an electric heater disposed near a sheet of paper fed from the developing section serves to heat and melt the toner to fix it on the paper. In this type of device, the heater and the paper do not contact each other, and the heating of the toner is made by heat radiated from the electric heater. It is therefore necessary that the temperature of the heater be cosiderably higher than that sufficient for melting the toner, and hence a relatively wide range of temperature is permitted. This makes the temperature control of the heater easier. However, the oven type device has the disadvantages that the fixing section consumes a large amount of electric power and that there is always a danger of fire because of the high temperature of the heater.
The fixing section provided with the heating roller type device eliminates such disadvantages. This type of device is arranged such that a toner-bearing paper passes through the nip between a heating roller heated at a predetermined temperature and a pressing roller to fix the toner image to the paper by directly contacting the paper with the heating roller. With this arrangement, the heating roller type device has the advantage that the thermal efficiency is very high and hence the power consumption is very low. However, it has the disadvantage that the allowable range of temperature is very narrow, so that when the temperature of the heating roller deviates from this range, such adverse effects as insufficient or defective fixing or the offset phenomenon may occur.
In view of this, the conventional heating roller type device is provided with a temperature control circuit having a thermosensitive element to control the temperature of the heating roller always within a predetermined range. The heater and the thermosensitive element are provided inside the heating roller which constitutes a rotating body, so that electrical connections between them and external circuits are effected by means of sliding contact mechanisms each of which comprises an electrically conductive fixed brush and an electrically conductive ring rotatable relative thereto in sliding contact therewith. Such an electrical connection is preferable in that the structure is simple. However, it has the disadvantage that insufficient contact between the brush and the ring is apt to occur owing to the wear of the brush, dust lying between them, or the like. In particular, when the signal from the thermosensitive element is interrupted by the insufficient contact with the result that the electric control is impaired, the heater becomes overheated. As a result, the offset phenomenon, in which the toner is stuck on the heating roller occurs, producing an unsatisfactory picture and in the worst case a fire may be caused.